/*
 * Copyright 2016 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.FutureListener;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.PlatformDependent;
import io.netty.util.internal.UnstableApi;

import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * A {@link NameResolver} that resolves {@link InetAddress} and force Round
 * Robin by choosing a single address randomly in {@link #resolve(String)} and
 * {@link #resolve(String, Promise)} if multiple are returned by the
 * {@link NameResolver}. Use {@link #asAddressResolver()} to create a
 * {@link InetSocketAddress} resolver
 */
@UnstableApi
public class RoundRobinInetAddressResolver extends InetNameResolver
{
    private final NameResolver<InetAddress> nameResolver;

    /**
     * @param executor the {@link EventExecutor} which is used to notify the
     *        listeners of the {@link Future} returned by
     *        {@link #resolve(String)}
     * @param nameResolver the {@link NameResolver} used for name resolution
     */
    public RoundRobinInetAddressResolver(EventExecutor executor,
            NameResolver<InetAddress> nameResolver)
    {
        super(executor);
        this.nameResolver = nameResolver;
    }

    @Override
    protected void doResolve(final String inetHost,
            final Promise<InetAddress> promise) throws Exception
    {
        // hijack the doResolve request, but do a doResolveAll request under the
        // hood.
        // Note that InetSocketAddress.getHostName() will never incur a reverse
        // lookup here,
        // because an unresolved address always has a host name.
        nameResolver.resolveAll(inetHost)
                .addListener(new FutureListener<List<InetAddress>>()
                {
                    @Override
                    public void operationComplete(
                            Future<List<InetAddress>> future) throws Exception
                    {
                        if (future.isSuccess())
                        {
                            List<InetAddress> inetAddresses = future.getNow();
                            int numAddresses = inetAddresses.size();
                            if (numAddresses > 0)
                            {
                                // if there are multiple addresses: we shall
                                // pick one by one
                                // to support the round robin distribution
                                promise.setSuccess(inetAddresses
                                        .get(randomIndex(numAddresses)));
                            }
                            else
                            {
                                promise.setFailure(
                                        new UnknownHostException(inetHost));
                            }
                        }
                        else
                        {
                            promise.setFailure(future.cause());
                        }
                    }
                });
    }

    @Override
    protected void doResolveAll(String inetHost,
            final Promise<List<InetAddress>> promise) throws Exception
    {
        nameResolver.resolveAll(inetHost)
                .addListener(new FutureListener<List<InetAddress>>()
                {
                    @Override
                    public void operationComplete(
                            Future<List<InetAddress>> future) throws Exception
                    {
                        if (future.isSuccess())
                        {
                            List<InetAddress> inetAddresses = future.getNow();
                            if (!inetAddresses.isEmpty())
                            {
                                // create a copy to make sure that it's
                                // modifiable random access collection
                                List<InetAddress> result = new ArrayList<InetAddress>(
                                        inetAddresses);
                                // rotate by different distance each time to
                                // force round robin distribution
                                Collections.rotate(result,
                                        randomIndex(inetAddresses.size()));
                                promise.setSuccess(result);
                            }
                            else
                            {
                                promise.setSuccess(inetAddresses);
                            }
                        }
                        else
                        {
                            promise.setFailure(future.cause());
                        }
                    }
                });
    }

    private static int randomIndex(int numAddresses)
    {
        return numAddresses == 1 ? 0
                : PlatformDependent.threadLocalRandom().nextInt(numAddresses);
    }
}
